Whiteflies, mites, aphids, thrips, mealybugs, and other pests cause millions of dollars of damage each year to ornamental plants and plants grown in greenhouses. For example, the sweet potato whitefly Bemisia tabaci (Gennadius) has appeared on poinsettias in California, Florida, Georgia, and North Carolina. During 1981, B. tabaci was responsible for crop and market losses of $100 million in cotton, cucurbits, and lettuce in California and Arizona. The whitefly is increasingly a problem in Florida where, in 1986, B. tabaci caused approximately $2 million of damage to Florida's $8 to 10 million poinsettia crop.
B. tabaci is also a pest of international importance, having been found on host plants throughout the mid-east Caribbean and Central America. This insect is now known to feed on more than 500 different plants, many of which are of importance in the Caribbean and Florida. For example, cassava, sweet potato, squash, tomato, beans, lettuce, cotton, pepper, carrot, cucumber, egg plant, and watermelon are all known hosts. This species of whitefly severely impacts infested plants by its feeding, production of honeydew with resultant growth of sooty mold, and transmission of plant pathogens. Most extensive losses to this pest have been through direct feeding damage and indirect damage through transmission of plant diseases.
Whitefly borne diseases are of major importance in tropical and subtropical agriculture. More than 70 diseases caused by viruses and microorganisms are known to be transmitted by whiteflies, with most of them being transmitted by B. tabaci. In Puerto Rico, this whitefly is a vector of at least seven diseases. One of these diseases is the bean golden mosaic virus, a disease affecting many legumes.
B. tabaci (Gennadius) has proven to be very difficult to control with conventional pesticide applications. Many factors contribute to the lack of control obtained with pesticides. The most important factor is that this whitefly has demonstrated a broad spectrum of resistance to chlorinated hydrocarbon, organophosphorus, carbamate, and synthetic pyrethroid insecticides. Very few commercially available pesticides are effective against whiteflies, and those which do work are only effective if care is taken to make a very thorough application of the insecticide several times a week. The sweet potato whitefly spends most of its life on the undersides of leaves, therefore, growers must adjust their management practices to permit increased pesticide coverage there. The spacing of the plants must be such that the chemical spray can penetrate the canopy and reach all surfaces of the plants.
In addition to being largely ineffective, and difficult and costly to apply, chemical control of these pests has other significant drawbacks. For example, the use of chemical pesticides presents the further disadvantages of polluting the environment, creating potential health hazards to agricultural workers and to consumers, development of resistance to chemicals in target pest species, detrimental effect of these chemicals on nontarget species resulting in secondary pest outbreaks, and phytotoxic reactions by treated plants.
Because of the problems associated with the use of chemical pesticides, safer and more effective methods of control for pests are clearly needed. Although biological control agents are actively being sought-after, to date no biological control agent has been commercially successful for the control of this whitefly.
Biological control agents are needed not only for B. tabaci, but also for other common pests of greenhouse and ornamental plants. These other common pest include mites, thrips, mealybugs, aphids, and scales. Twospotted spider mites, for example, feed on many species of plants and are a major pest of vegetables, ornamentals, fruit trees, hops, cotton, and strawberries. It is believed that widespread use of broad-spectrum insecticides destroy or greatly hamper natural enemies of spider mites and may thereby allow pest outbreaks to occur.
Biological control agents have been tried for many, if not all, of these pests. However, availability, limited host range, cost and reliability have reduced the potential for implementing the use of these biological control agents. The development of broad spectrum mycoinsecticides will reduce the need for many of the petrochemically based pesticides. By using fungi to control pests, the potential for resistance development is minimized, which, in turn, will stabilize many of the pest management programs.